L4D was somewhat, idk, arcade-y in a fashion. You have 4 stock characters dropped in a map full of 'infected', some of them special. These special infected are controlled by players in versus mode, and it's them that hold up the strategic gameplay in L4D. The human characters generally just push through and take care with their equipment during horde rushes, while special infected have to focus on placing, timing, and other factors. Characters either finish the level or die.
Evolve sort of turns things on the head by making everything strategic. You have diverse character classes with their own varied sub-types, as well as the 3 (4 including DLC) monsters who so far are very different from each other. Movement is far more varied, and the matches are more open in nature while still focusing the combat more than in L4D. The monster's goal is to feed and grow, putting limited points to certain skills. It has to carefully take apart the Hunters, and is generally going to be the only source of conflict rather than having the cover of other adversaries as in L4D. Monster will also be given an alternative objective to complete to win the match.
The Hunters of course have to hunt the monster down, as early as possible to avoid a tougher and more deadly creature. Everyone has jobs and communication is paramount, in order to take advantage of each of their abilities. Outside of the monster, they have to take care with dangerous fauna around the map. Hunters can respawn given certain time, other than the monster who gets only one life.
So differences are:
- Asymetrical, strategic gameplay across both factions.
- Varied gameplay templates across each faction
- Open match design, rather than restricting spawns and movement to where opposing faction is.
- Varied objectives to allow for alternative match progression.
- Varied gameplay across classes, subtypes, monsters, and monster point allocation keep matches from being repetitive affairs within each map and game mode.
There's probably more, but I didn't see much of the E3 gameplay.